A turbopropeller conventionally includes:                a turbomachine including a combustion chamber, and enclosures, in particular enclosures for roller bearings,        a propeller enclosure including a speed reducer,        at least one propeller rotatably driven by the turbomachine,        a pitch actuating system for managing the pitch setting of the propeller vanes.        
The various enclosures and the pitch actuating system are lubricated via a lubrication system including an oil feed device, the feed device comprising at least one feed pump. The lubrication system also includes an assembly of scavenge pumps to scavenge oil downstream of the enclosures and the pitch actuating system, as well as exchangers to cool oil by heat exchange with fuel or air.
The terms upstream and downstream are to be taken into consideration with respect to an oil flow direction from the feed device to the enclosures and the pitch actuating system, wherein the oil flows from upstream to downstream.
A representative scheme of a first lubrication system 10 of a turbopropeller according to prior art is illustrated in FIG. 1. The lubrication system 10 includes an oil feed device 11 comprising a feed pump 15. The lubrication system also includes an assembly of scavenge pumps and heat exchangers 12. The lubrication system 10 is used to lubricate at least one enclosure 13 and a pitch actuating system 14. The enclosure 13 and the pitch actuating system 14 are thus connected to the feed pump 15 of the feed device 11. The feed pump 15 is a displacement pump which will be referred to as “feed pump 15” hereinafter. Part of the flow rate is used to lubricate the enclosure 13, another part is used to lubricate the pitch actuating system 14: the enclosure 13 and the pitch actuating system 14 are said to be fed in parallel. In the case of a plurality of enclosures to be lubricated, the enclosures are fed in parallel two by two, and the total flow rate of the feed pump 15 is shared between the multiple enclosures 13 and the pitch actuating system 14.
It is noted that the pitch actuating system 14 is an equipment operating under high pressure. The pressure at the pitch actuating system 14 is higher than the pressure at an enclosure 13. Thus, a pump 16 located upstream of the pitch actuating system 14, referred to as “pump of the pitch actuating system 16”, supplies oil at the pitch actuating system 14. The pump of the pitch actuating system 16 is a displacement pump and is placed between the feed pump 16 and the pitch actuating system 14. The pump of the pitch actuating system 16 is a displacement pump, its flow rate is sized to the maximum flow rate potentially required by the pitch actuating system 14.
A pressure restricting valve 17 placed in parallel to the pump of the pitch actuating system 16 enables the oil flux to be redirected from downstream of the pump of the pitch actuating system 16 to upstream of the pump of the pitch actuating system 16. This valve 17 is partly or wholly closed when the pitch actuating system 14 is active, that is when a pitch change is demanded, to enable the pitch actuating system to be lubricated. On the contrary, the valve 17 is wholly opened when the pitch actuating system 14 is inactive to enable the oil flux to be redirected.
On the other hand, the sizing of the feed pump 15, that is the calibration of the oil flow rate of the feed pump 15, is performed hypothesizing that there are simultaneous maximum flow rates in the enclosure 13 and in the pitch actuating system 14. This results in the following problem; when the pitch actuating system 14 is inactive, the pump of the pitch actuating system 16 operates all the same and the oil flow rate provided for the pitch actuating system 14 is redirected upstream of the pump of the pitch actuating system 16. The oil flow rate pumped by the feed pump 15 provided for the pitch actuating system 14 is thus redirected to the enclosures 13, in addition to the flow rate provided for the enclosures 13. This overflow rate creates warming of oil in the enclosure 13 through churning, which makes oversizing of the heat exchanger 12 necessary.
A second lubrication system 20 according to prior art is illustrated in FIG. 2, Elements identical to those of the first lubrication system 10 bear the same reference numerals as in FIG. 1. The second lubrication system 20 enables overflow rate drawbacks of the previously described first lubrication system 10 to be avoided. The feed device 21 of the second lubrication system 20 includes two feed pumps 22, 23. The first feed pump 22 is a displacement pump sized on the maximum flow rate potentially required by the enclosure 13, whereas the second feed pump 23 is a centrifugal pump dedicated to the feed of the pitch actuating system 14. Two distinct feed branches 24, 25 are thus used to lubricate the pitch actuating system 14 and the enclosure 13, which avoids overflow rate problems in the enclosure 13 when the pitch actuating system 14 is inactive.
However, the second lubrication system 20 comprises a further feed pump and further pipings, which negatively impacts the mass and volume of the system.